Facile one-pot synthesis of a NiMoO₄/reduced graphene oxide composite as a pseudocapacitor with superior performance

Abstract

A hybrid NiMoO₄/rGO composite was successfully synthesized by a facile one-pot hydrothermal method. A series of characterization techniques: SEM, TEM, XRD, Raman, XPS and N₂ adsorption/desorption isotherms were used to verify the special nanostructure and unique composition of the as-prepared products, in which the NiMoO₄ nanowires are homogenously distributed in the interconnected rGO network, and at the same time the NiMoO₄ nanowires are encapsulated within the rGO sheets, further preventing the rGO nanosheets from agglomerating and restacking. As an electrode material for pseudocapacitors, the NiMoO₄/rGO composite exhibited a maximum specific capacitance of 1202 F g⁻¹ at 1 A g⁻¹, and still remained as high as 775 F g⁻¹ at 10 A g⁻¹ and 592 F g⁻¹ at 20 A g⁻¹, which are superior to those of pure NiMoO₄ nanowires. The enhanced capacitive performance of the as-prepared NiMoO₄/rGO composite is closely related to the form of an internal mesoporous structure by utilizing the high electrical conductivity and large surface area of rGO nanosheets, which could provide more electroactive sites, a larger electrode–electrolyte contact area and accelerate effective ion and electron transport in the whole electrode. The results suggest that such a hybrid electrode has great potential applications in high performance energy-storage systems.

---

1 Introduction

Supercapacitors are considered as one of the most promising candidates for next generation power devices owing to their ultralong cycling stability, high power density and fast charge/discharge rate.^1–3 Generally, on the basis of the energy storage mechanism, supercapacitors can be classified into the electrical double layer capacitors (EDLCs) and pseudocapacitors. EDLCs only involve physical adsorption of ions without any chemical reactions. Carbon materials with high surface areas, such as activated carbon, carbon nanotube and graphene, generally display double-layered capacitive behaviour, and are usually highly conductive and maintain particularly stable during the long running process.^4–6 But for pseudocapacitors, the energy storage capability is determined by the fast and reversible faradic reaction near the surface.⁷ Transition metal compounds, including oxides,^8–11 hydroxides,^12–14 sulfides,^15–17 phosphides,^18,19 and nitrides^20–22 are important electrode materials for pseudocapacitors, which usually exhibit larger specific capacitance than EDLCs and have attracted much attention in recent years. Among these materials, the transition metal molybdates, such as NiMoO₄,^23–25 CoMoO₄ (ref. 26 and 27) and MnMoO₄,^28,29 have received considerable research attention due to their low cost, abundant resources, high redox activity, well-defined redox behaviour and environmental compatibility. However, some challenges are still obstructing the application of the reported electrode materials based on transition metal oxide, and one of the major drawbacks is their low electrical conductivity.³⁰ Therefore, considering the high conductivity of carbonaceous materials, the pseudocapacitors can be commercialized in hope of a perfect combination of these transition metal materials with the high-surface-area carbon materials is expected.^31–33

Recently, graphene has been considered as an ideal electrode material for supercapacitor because of its outstanding electronic properties, good chemical stability and high surface area.^34–36 Previous studies have exhibited that graphene (or reduced graphene oxide (rGO)) can be combined with different kinds of transition metal compounds and used as high performance supercapacitors.^37–44 Herein, we prepared nanocomposites consisting of NiMoO₄ nanowires and conductive rGO sheets using a simple one-pot hydrothermal method. In this composite, the rGO nanosheets function as the substrate upon which to deposit the NiMoO₄ nanowires. Besides, the rGO improves not only the electrical conductivity of the hybrids, but also the effective utilization of the active material. As a result, the hybrid composite possesses a huge specific capacitance and superior cycling stability, providing with a prospective candidate for high-performance pseudocapacitors.

2 Experimental

2.1 Synthesis of NiMoO₄ nanowires on graphene

Graphene oxide (GO) was synthesized from natural graphite flake by a modified Hummer's method.⁴⁵ The synthesis of NiMoO₄ nanowires on rGO was carried out through one-pot hydrothermal process. In a typical synthesis, 10 mg of GO was dispersed in 10 mL deionized water under ultrasonication and then 0.25 g Ni(CH₃COO)₂·4H₂O was dissolved into the GO suspension under vigorous magnetic stirring at room temperature. After stirring for 1 h, 0.2 g (NH₄)₆Mo₇O₂·4H₂O was added to the solution and the mixture was stirred for another 1 h in a water bath of 65 °C. Then the mixture was transferred into a 25 mL Teflon-lined stainless steel autoclave and kept in 160 °C for 10 h. The product was washed with deionized water and ethanol for several times and finally dried in a vacuum at 60 °C for 24 h. The as-obtained samples were denoted as NiMoO₄/rGO. In addition, according to the same method without adding GO, the pure NiMoO₄ nanowires was prepared to be as a contrast. And for comparison, rGO was also synthesized with the same process without the additions of Ni(CH₃COO)₂·4H₂O and (NH₄)₆Mo₇O₂·4H₂O. Meanwhile, the mechanical mixture sample of NiMoO₄ nanowires and rGO (simplified as NiMoO₄ + rGO) was also synthesized at same composition ratio with the NiMoO₄/rGO composite prepared by one-pot hydrothermal process.

2.2 Characterizations

X-ray diffraction (XRD) patterns were recorded on a Bruker D8 advance diffractometer, using Cu-Kα radiation at 40 kV and 40 mA. Raman measurements were performed on a Horiba Jobin Yvon LabRAM HR800 confocal micro-Raman spectrometer with incident laser light of 532 nm. X-ray photoelectron spectroscopy (XPS) was carried out by using Thermo Scientific ESCALAB 250XI spectrometer. Nitrogen adsorption/desorption isotherms were measured on ASAP 2020 analyser. The nanostructured samples were observed by using a field emission scanning electron microscope (SEM, JEOL JSM-7001F) and a field emission transmission electron microscope (TEM, FEI Tecnai G20).

2.3 Electrochemical measurements

The electrochemical measurements were performed using a three-electrode system in 2 M KOH aqueous electrolyte at room temperature. The fabrication of working electrode was carried out as follows. The as-synthesized electroactive material was mixed with carbon black and polytetrafluoroethylene (PTFE) in a mass ratio of 85 : 10 : 5 in ethanol to produce a homogeneous paste. Then the paste was coated onto a piece of nickel foam (≈1 cm²), and then dried and pressed at a pressure of 10 MPa to give the working electrode. The mass loading of the active materials on Ni foam was around 1.0 mg cm⁻² for pure NiMoO₄ and NiMoO₄ + rGO mechanical mixture, and 0.8 mg cm⁻² for hybrid NiMoO₄/rGO composite. A platinum foil (1.5 × 1.5 cm²) and a saturated calomel electrode (SCE) served as the counter and reference electrodes respectively. Cyclic voltammetry (CV), galvanostatic charge–discharge and electrochemical impedance spectroscopy (EIS) measurements were carried out on the Autolab PGSTAT302N electrochemical workstation. Impedance spectroscopy tests were performed at open circuit potential with a potential amplitude of 5 mV in the frequency range from 100 kHz to 0.01 Hz. The specific capacitance of the electrode were calculated according to the following equations:where C is the specific capacitance of the electroactive materials (F g⁻¹), i is the discharging current density (A g⁻¹), t is discharging time (s) and ΔV is the discharging potential (V).

3 Results and discussion

Fig. 1 briefly illustrates the fabrication of NiMoO₄/rGO composite through a simple one-pot hydrothermal process. When the Ni(CH₃COO)₂·4H₂O was mixed with the GO suspension, metal cation of Ni²⁺ was adsorbed on the surface of GO due to the electrostatic attraction between the Ni²⁺ and the negatively charged oxygen-containing functional groups of GO, such as C–OH and C–O–C. Followed by adding (NH₄)₆Mo₇O₂·4H₂O, the mixture was transferred into an autoclave and heated at 160 °C for 10 h. The reduction of GO and the in situ formation of NiMoO₄ were achieved simultaneously, causing the generation of NiMoO₄/rGO.

Fig. 1 Schematic illustration of the fabrication process of hybrid NiMoO₄/rGO composite.

The morphologies of rGO, pure NiMoO₄ and hybrid NiMoO₄/rGO were investigated by SEM and TEM. In Fig. 2a, the rGO shows a sheet-like morphology with characteristic rough and crumpled architecture due to its high flexibility and strong interaction between nanosheet layers. For NiMoO₄/rGO composite (Fig. 2c), the NiMoO₄ nanowires are homogenously distributed in the interconnected rGO network compared to pure NiMoO₄ sample (Fig. 2b). Meanwhile, as seen from the TEM image in Fig. 2e, the NiMoO₄ nanowires are encapsulated within the rGO sheets, which can efficiently prevent the rGO nanosheets from agglomerating and restacking. Such a nanostructured surface morphology can provide more voids among the interconnected NiMoO₄ nanowires, resulting in improving contact area and shortening diffusion path between the electrode material and the electrolyte.⁴⁶ From the HRTEM image presented in Fig. 2f, the ambiguous interface can be observed between NiMoO₄ and rGO. The distinguishable lattice spacing of 0.214 nm is attributed to the (121) plane of NiMoO₄ in the hybrid structure,⁴⁷ and the crinkled lattice fringes along the edge of NiMoO₄ nanowire correspond to the rGO nanosheet layers.⁴⁸

Fig. 2 SEM images of (a) rGO, (b) pure NiMoO₄ nanowires and (c) hybrid NiMoO₄/rGO composite; (d) SEM image of NiMoO₄/rGO composite as electrode after cycling test; (e) TEM and (f) HRTEM images of NiMoO₄/rGO composite.

XRD was used to identify the as-prepared samples and the results are shown in Fig. 3a. For GO, the diffraction peak at 2θ = 10.6° is corresponded with the (002) interlayer spacing of 0.83 nm, indicating the deep oxidation of graphene and the introduction of the oxygen-containing functional group.⁴⁹ For rGO, the peak at 10.6° disappears and a new broad diffraction peak at about 2θ = 25° appears, implying the GO is readily reduced after hydrothermal treatment at 160 °C for 10 h.⁵⁰ The strong diffraction peaks of NiMoO₄/rGO are observed at 2θ = 11.0, 27.2, 29.7 and 33.2°, which is in good accordance with the standard diffraction pattern of NiMoO₄·xH₂O (JCPDS card no. 13-0128).⁵¹ On the whole the peak intensities for NiMoO₄ in hybrid NiMoO₄/rGO are relatively weak compared with those in pure NiMoO₄ nanowires. However, the peak intensity at 2θ = 27.2° in NiMoO₄/rGO composite is similar with that in pure NiMoO₄ nanowires, which is due to the broad diffraction peak at 2θ = 20–30° of rGO and verifies the existence of rGO in the hybrid composite. So the XRD analysis proves the successful preparation of NiMoO₄/rGO.

Fig. 3 (a) XRD patterns and (b) Raman spectra of GO, rGO, pure NiMoO₄ nanowires and hybrid NiMoO₄/rGO composite.

The Raman spectra of GO, rGO, pure NiMoO₄ and hybrid NiMoO₄/rGO are shown in Fig. 3b. The Raman spectrum of GO or rGO exhibits a G band at 1601 cm⁻¹ which is related to the vibration of sp²-bonded carbon atoms in two degree hexagonal lattice, and a D band at 1357 cm⁻¹ which corresponds to the defects and disorder in the hexagonal graphitic layer.^52,53 For pure NiMoO₄ nanowires, the Raman spectrum shows an intense peak at 945 cm⁻¹ along with some medium intensity peaks appearing at 860, 825 and 357 cm⁻¹, which can be attributed to the characteristic peaks of NiMoO₄.^54,55 The main peaks of rGO and NiMoO₄ are all observed in the Raman spectrum of NiMoO₄/rGO composite again verifying the successful synthesis of hybrid NiMoO₄/rGO composite.

The intensity ratio of the D to G band (I_D/I_G) can be used to estimate the degree of defect in carbonaceous material.⁵⁶ As shown in Fig. 3b, the I_D/I_G value of rGO (1.05) is higher than that of GO (0.98), indicating the rGO has a small size and a large quantity of edges which act as defects and lead to an increased D peak.^57,58 In addition, compared with the pure rGO (1.05), a higher I_D/I_G ratio is obtained for the hybrid NiMoO₄/rGO (1.09), suggesting that the rGO was greatly reduced and formed a high level disordered structure during the hydrothermal process in the presence of Ni²⁺ or NH₄⁺. As we know, increasing the defect and disorder of rGO can provide more active sites for electron storage and enhance the binding energy with ions in electrolyte.⁵⁹ So the as-synthesized NiMoO₄/rGO composite is expected to have a promising application as an efficient electrode for high-performance pseudocapacitors.

Information on the chemical composition of the as-prepared hybrid is obtained from XPS measurement, just as shown in Fig. 4 and Table 1. The XPS survey spectrum of NiMoO₄/rGO (Fig. 4b) indicates four dominant elements: C, O, Ni and Mo with atomic percentages of 32.24, 47.05, 10.59 and 9.94% respectively. The atomic ratio of Ni : Mo is 1.06 : 1, which is close to the stoichiometry of NiMoO₄, further proving the formation of NiMoO₄. As seen in Table 1, because the O in hybrid NiMoO₄/rGO is from both MoO₄²⁻ and rGO, the percentage of O in rGO is 7.29% (47.05% minus 39.76%). The ratio of C/O in hybrid NiMoO₄/rGO (4.42) is higher than that in pure GO (2.10), indicating again the oxygen-containing functional groups were removed after the hydrothermal process.³⁹ The mass amount of NiMoO₄ in the hybrid NiMoO₄/rGO composite calculated from the XPS data is 81.5%.

Fig. 4 (a and b) XPS survey spectra of pure GO and hybrid NiMoO₄/rGO composite. (c and d) C 1s spectra of pure GO and hybrid NiMoO₄/rGO. (e) Ni 2p spectrum and (f) Mo 3d spectrum of hybrid NiMoO₄/rGO composite.

Table 1 Elemental analysis of hybrid NiMoO₄/rGO and pure GO based on XPS results

Atomic (%)	C	O	Ni	Mo	C/O
Hybrid NiMoO4/rGO	32.24	47.05	10.59	9.94	—
NiMoO4 in hybrid		39.76	10.59	9.94	—
rGO in hybrid	32.24	7.29	—	—	4.42
Pure GO	67.79	32.21	—	—	2.10

---

The C 1s core level XPS spectra could be deconvoluted into three synthetic peaks, just as seen in Fig. 4c and d. The peaks centered at 284.6, 286.0 and 288.3 eV are assigned to the sp² carbon atoms (C–C/C=C), hydroxyl/epoxy groups (C–O) and carbonyl group (C=O) respectively.⁵⁷ The peak intensity of C–O in NiMoO₄/rGO decreases dramatically compared with that in GO, which further indicates that the C–O–C and C–OH groups were selectively occupied by Ni²⁺ or Mo₇O₂⁶⁻ and then reduced during the one-pot hydrothermal synthesis.⁵⁷

For high resolution scan of Ni 2p (see Fig. 4e), there are two main peaks at 856.0 and 873.8 eV assigned to Ni 2p_3/2 and Ni 2p_1/2 respectively, with a spin energy separation of 17.8 eV. Meanwhile the satellite peaks (Ni 2p_3/2 satellite at 862.3 eV and Ni 2p_1/2 satellite at 880.5 eV) are also observed, which are associated with the Ni²⁺ oxidation state.⁵⁸ The Mo 3d core level spectrum (Fig. 4f) shows two peak at 232.4 and 235.5 eV, corresponding to Mo 3d_5/2 and Mo 3d_3/2 respectively with a spin energy separation of 3.1 eV, which signifies a Mo⁶⁺ oxidation state.⁵⁴ So the XPS results again proves the formation of NiMoO₄/rGO composite.

The electrochemical performance of the as-prepared samples partially depends on their specific surface area and the porous structure, which were characterized by nitrogen adsorption/desorption measurement. As see in Fig. 5, a type IV isotherm plot with a distinct hysteresis loop for the pure NiMoO₄ and hybrid NiMoO₄/rGO occurs in the range of 0.6–1.0 p/p₀, indicating the existence of mesopores in the materials. The corresponding pore size distribution derived from the adsorption branches of isotherms in the inset of Fig. 5, further confirms the mesoporous nature of the samples by the sharp peaks in the pore size range of 2–6 nm. Meanwhile, compared to the pure NiMoO₄ (0.056 cm³ g⁻¹), the hybrid NiMoO₄/rGO composite shows a more Barrett–Joyner–Halenda (BJH) mesoporous volume of 0.072 cm³ g⁻¹, which can be attributed to the presence of ultrathin rGO nanosheets. These ultrathin rGO nanosheets create voids between adjacent NiMoO₄ nanowires that increase the surface area. So the calculated Brunauer–Emmett–Teller (BET) surface area of the NiMoO₄/rGO was 58.1 m² g⁻¹, which was higher than that of pure NiMoO₄ nanowires (32.1 m² g⁻¹). This mesoporous structure with higher specific surface area and more mesopore volume has potential applications in pseudocapacitors, as it provides more electroactive sites and larger interface of active electrode materials with electrolyte solution.⁶⁰

Fig. 5 Nitrogen adsorption/desorption isotherm plots and Barrett–Joyner–Halenda (BJH) pore size distribution of (a) pure NiMoO₄ nanowires and (b) hybrid NiMoO₄/rGO composite.

The electrochemical properties of as-synthesized materials as electrodes were explored in a three-electrode system. The typical cyclic voltammetry (CV) curves of pure NiMoO₄ nanowires, hybrid NiMoO₄/rGO composite and NiMoO₄ + rGO mechanical mixture within the potential window of 0–0.6 V at various scan rates are shown in Fig. 6a, c and e. All are observed a couple of well-defined redox peaks, indicating the capacitance characteristics are mainly governed by faradaic redox reactions, which possibly corresponds to the reversible conversion between Ni³⁺/Ni²⁺.⁵⁴ Besides, in contrast to the CV curve of pure NiMoO₄ nanowires and NiMoO₄ + rGO mechanical mixture, the hybrid NiMoO₄/rGO electrode exhibits larger enclosed CV curve area at the same scan rate (as seen in Fig. 7a), indicating higher capacitance. This is in good accordance with the results obtained by the galvanostatic charge–discharge measurement in Fig. 6b, d and f. The specific capacitance of the electrodes calculated from the discharge curves are shown in Fig. 7b. The capacitance value of NiMoO₄/rGO composite electrode is 1202 F g⁻¹ at the current density of 1 A g⁻¹, which is much higher than that of pure NiMoO₄ (873 F g⁻¹) and NiMoO₄ + rGO mechanical mixture at the same current density. Meanwhile, the specific capacitance of pure NiMoO₄ and hybrid NiMoO₄/rGO decreases gradually with increasing the current density. However, the hybrid NiMoO₄/rGO still exhibits excellent pseudocapacitance of 903, 775, 653 and 592 F g⁻¹ at the high current densities of 6, 10, 15 and 20 A g⁻¹ respectively. For comparison, the capacitance of pure NiMoO₄ is only 493, 392, 320 and 267 F g⁻¹ at the same high current densities of 6, 10, 15 and 20 A g⁻¹ respectively. And the capacitances of NiMoO₄ + rGO mechanical mixture were just similar with those of pure NiMoO₄. Impressively, these results are remarkable as compared to the previous reported NiMoO₄ and graphene based composites. For example, the specific capacitance of the graphene decorated with 1D NiMoO₄·nH₂O nanorods is 312 F g⁻¹ at 5 A g⁻¹;⁵⁴ the mesoporous NiMoO₄ nanorod/rGO composite prepared by a microwave solvothermal method exhibits much lower specific capacitance of 573 F g⁻¹ at high current density of 10 A g⁻¹ although its initial capacitance is 1274 F g⁻¹ at 1 A g⁻¹.⁶¹ The clearly enhanced capacitive performance of hybrid NiMoO₄/rGO composite is closely related to the excellent electrical conductivity of the rGO sheets in the composite, which can act as the current collector to decrease the electrode resistance. Besides, the large surface area of rGO sheets facilitates the uniform growth of NiMoO₄ nanowires, and on the other hand the well-dispersed NiMoO₄ nanowires further prevent the agglomerating and restacking of rGO nanosheets. Therefore, the uniform nanostructure of as-prepared NiMoO₄/rGO composite can provide a larger electrode–electrolyte contact area, improve the electrochemical utilization of active material, shorten the diffusion length for adsorbing ions, and accelerate the electron transfer during the charge–discharge process.^40,62

Fig. 6 (a, c and e) CV curves at different scan rates and (b, d and f) galvanostatic charge–discharge curves at different current densities for pure NiMoO₄ nanowires, hybrid NiMoO₄/rGO composite and NiMoO₄ + rGO mechanical mixture electrodes.

Fig. 7 (a) CV curves of pure NiMoO₄ nanowires, hybrid NiMoO₄/rGO composite and NiMoO₄ + rGO physical mixture electrodes at a scan rate of 5 mV s⁻¹; (b) specific capacitance of pure NiMoO₄ nanowires, hybrid NiMoO₄/rGO composite and NiMoO₄ + rGO mechanical mixture electrodes as a function of current densities.

Fig. 8a shows the cycling performance of the pure NiMoO₄ nanowires, hybrid NiMoO₄/rGO composite and NiMoO₄ + rGO mechanical mixture at current density of 20 A g⁻¹. The specific recharge capacitance of NiMoO₄/rGO composite maintains 66.1% in contrast with 57.1% capacitance retention of pure NiMoO₄ for 3000 cycles under same experimental condition, indicating that the rGO significantly improves the cyclic stability of NiMoO₄ in hybrids. Meanwhile capacitance retention of synthesized NiMoO₄/rGO composite was also higher than that of NiMoO₄ + rGO mechanical mixture. In addition, for NiMoO₄/rGO electrode, the specific capacitance decreases about 30% of the initial capacitance during the first 1000 cycles, which can be generally explained by the irreversible of the Faraday reactions or the dissolution of active materials from the hybrid nanostructures at the beginning of the cycle test.⁶³ Then it only shows a slight decrease of 4% after the rest 2000 cycles, exhibiting relatively good stability. And from the SEM image of NiMoO₄/rGO electrode material after cycling test in Fig. 2d, it showed that cycling test has little effect on the morphology of the composite, indicating that the structure of NiMoO₄ nanowires on rGO nanosheets is still maintained.

Fig. 8 (a) Cycling performance at 20 A g⁻¹ and (b) EIS spectra for pure NiMoO₄ nanowires, hybrid NiMoO₄/rGO composite and NiMoO₄ + rGO mechanical mixture electrodes.

To further understand the superior performance of the NiMoO₄/rGO composite, the kinetic feature of the corresponding electrode was investigated using electrochemical impedance spectroscopy (EIS) in 2 M KOH solution. Fig. 8b shows the Nyquist plots extracted from the EIS measurements on pure NiMoO₄ nanowires, hybrid NiMoO₄/rGO composite and NiMoO₄ + rGO mechanical mixture. It can be seen that all impedance spectra are composed of a semicircle at high frequency and a straight line at low frequency. At the high frequency region, the semicircle is corresponding to faradic resistance caused by the interfacial electron transport in electroactive material, and the diameter of the semicircle reflects the electron transfer resistance on the electrode surface.^54,64 As shown in Fig. 8b, the NiMoO₄/rGO composite exhibits much lower faradic resistance than pure NiMoO₄ and NiMoO₄ + rGO mechanical mixture, suggesting that the introduction of rGO by chemical hydrothermal method significantly improves the electrical conductivity of the composite. At the low frequency region, the slope of the line is associated with Warburg resistance resulted from ion diffusion in the electrolyte to the electrode interface.⁵⁷ Compared with pure NiMoO₄ and NiMoO₄ + rGO mechanical mixture, the NiMoO₄/rGO composite shows a more vertical curve in the low frequency region, indicating the fast ion diffusion in the electrolyte and adsorption onto the electrode surface. Therefore, within the as-prepared NiMoO₄/rGO composite, the introduction of rGO by hydrothermal method can dramatically improve the energy storage performance of NiMoO₄, by favouring the electron transfer of NiMoO₄ and increasing the efficient access of electrolyte ions.

4 Conclusions

In summary, we have successfully synthesized hybrid NiMoO₄/rGO composite using a facile one-pot hydrothermal process. In the composite, the NiMoO₄ nanowires are homogenously distributed into the interconnected rGO network, and at the same time the rGO nanosheets are prevented from agglomerating and restacking by encapsulating the NiMoO₄ nanowires into its sheet layers. Due to their special nanostructure and unique composition, the as-prepared NiMoO₄/rGO composite shows a great enhancement in electrical conductivity, specific capacitance and cycling stability by comparison with the pure NiMoO₄. Its maximum specific capacitance is 1202 F g⁻¹ at 1 A g⁻¹, and still as high as 775 F g⁻¹ at 10 A g⁻¹ and 592 F g⁻¹ at 20 A g⁻¹, which are comparable or superior to those so far reported for the graphene and NiMoO₄ hybrids in alkaline electrolyte. The excellent electrochemical performance of the hybrid NiMoO₄/rGO composite renders their application as a candidate electrode material for the high-performance pseudocapacitors of energy storage.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant No. 21576054), Guangdong Province Science and Technology Project (Grant No. 2016A020221033), Pearl River Nova Program of Guangzhou (Grant No. 2011J2200041), Guangdong Province Science and Technology Project (Grant No. 2012A030600006), Science and Technology Achievements Transformation Projects of Guangdong Higher Education Institutes (Grant No. cgzhzd1104).

References

1. J. R. Miller and P. Simon, Science, 2008, 321, 651–652.
2. M. Winter and R. J. Brodd, Chem. Rev., 2004, 104, 4245–4270.
3. P. Simon, Y. Gogotsi and B. Dunn, Science, 2014, 343, 1210–1211.
4. T. Lin, I. W. Chen, F. Liu, C. Yang, H. Bi, F. Xu and F. Huang, Science, 2015, 350, 1508–1513.
5. Y. Zhu, S. Murali, M. D. Stoller, K. J. Ganesh, W. Cai, P. J. Ferreira, A. Pirkle, R. M. Wallace, K. A. Cychosz, M. Thommes, D. Su, E. A. Stach and R. S. Ruoff, Science, 2011, 332, 1537–1541.
6. Y. Zhang, Z. Zhen, Z. Zhang, J. Lao, J. Wei, K. Wang, F. Kang and H. Zhu, Electrochim. Acta, 2015, 157, 134–141.
7. S. Liu, S. H. Sun and X. Z. You, Nanoscale, 2014, 6, 2037–2045.
8. M. Y. Ho, P. S. Khiew, D. Isa, T. K. Tan, W. S. Chiu and C. H. Chia, Nano, 2014, 9, 25.
9. K. Dai, L. Lu, C. Liang, J. Dai, Q. Liu, Y. Zhang, G. Zhu and Z. Liu, Electrochim. Acta, 2014, 116, 111–117.
10. X. Xu, L. Pei, Y. Yang, J. Shen and M. Ye, J. Alloys Compd., 2016, 654, 23–31.
11. Z.-D. Ni, Y.-T. Song, H.-Q. Chen and L.-Y. Lin, Electrochim. Acta, 2016, 190, 313–321.
12. J. Jiang, A. Zhang, L. Li and L. Ai, J. Power Sources, 2015, 278, 445–451.
13. J. Zhao, J. Chen, S. Xu, M. Shao, Q. Zhang, F. Wei, J. Ma, M. Wei, D. G. Evans and X. Duan, Adv. Funct. Mater., 2014, 24, 2938–2946.
14. Y. Wimalasiri, R. Fan, X. S. Zhao and L. Zou, Electrochim. Acta, 2014, 134, 127–135.
15. L. Mei, T. Yang, C. Xu, M. Zhang, L. Chen, Q. Li and T. Wang, Nano Energy, 2014, 3, 36–45.
16. Y. Ruan, J. Jiang, H. Wan, X. Ji, L. Miao, L. Peng, B. Zhang, L. Lv and J. Liu, J. Power Sources, 2016, 301, 122–130.
17. J.-J. Li, Y.-X. Hu, M.-C. Liu, L.-B. Kong, Y.-M. Hu, W. Han, Y.-C. Luo and L. Kang, J. Alloys Compd., 2016, 656, 138–145.
18. C. An, Y. Wang, Y. Wang, G. Liu, L. Li, F. Qiu, Y. Xu, L. Jiao and H. Yuan, RSC Adv., 2013, 3, 4628–4633.
19. X. Wang, W. Li, D. Xiong and L. Liu, J. Mater. Chem. A, 2016, 4, 5639–5646.
20. X. Lu, M. Yu, T. Zhai, G. Wang, S. Xie, T. Liu, C. Liang, Y. Tong and Y. Li, Nano Lett., 2013, 13, 2628–2633.
21. X. Lu, T. Liu, T. Zhai, G. Wang, M. Yu, S. Xie, Y. Ling, C. Liang, Y. Tong and Y. Li, Adv. Energy Mater., 2014, 4, 1300994.
22. W. Li, C.-Y. Cao, C.-Q. Chen, Y. Zhao, W.-G. Song and L. Jiang, Chem. Commun., 2011, 47, 3619–3621.
23. D. Guo, Y. Z. Luo, X. Z. Yu, Q. H. Li and T. H. Wang, Nano Energy, 2014, 8, 174–182.
24. Z. Yin, S. Zhang, Y. Chen, P. Gao, C. Zhu, P. Yang and L. Qi, J. Mater. Chem. A, 2015, 3, 739–745.
25. K. Xiao, L. Xia, G. Liu, S. Wang, L.-X. Ding and H. Wang, J. Mater. Chem. A, 2015, 3, 6128–6135.
26. D. Cai, B. Liu, D. Wang, L. Wang, Y. Liu, H. Li, Y. Wang, Q. Li and T. Wang, J. Mater. Chem. A, 2014, 2, 4954–4960.
27. X. Xia, W. Lei, Q. Hao, W. Wang and X. Wang, Electrochim. Acta, 2013, 99, 253–261.
28. G. K. Veerasubramani, K. Krishnamoorthy, R. Sivaprakasam and S. J. Kim, Mater. Chem. Phys., 2014, 147, 836–842.
29. D. Ghosh, S. Giri, M. Moniruzzaman, T. Basu, M. Mandal and C. K. Das, Dalton Trans., 2014, 43, 11067–11076.
30. B. Wang, S. Li, X. Wu, W. Tian, J. Liu and M. Yu, J. Mater. Chem. A, 2015, 3, 13691–13698.
31. Z. Yang, X. Zhou, Z. Jin, Z. Liu, H. Nie, X. Chen and S. Huang, Adv. Mater., 2014, 26, 3156–3161.
32. X. Xia, Y. Zhang, Z. Fan, D. Chao, Q. Xiong, J. Tu, H. Zhang and H. J. Fan, Adv. Energy Mater., 2015, 5, 1401709.
33. C. Yu, J. Yang, C. Zhao, X. Fan, G. Wang and J. Qiu, Nanoscale, 2014, 6, 3097–3104.
34. J. Zhu, D. Yang, Z. Yin, Q. Yan and H. Zhang, Small, 2014, 10, 3480–3498.
35. V. Chabot, D. Higgins, A. P. Yu, X. C. Xiao, Z. W. Chen and J. J. Zhang, Energy Environ. Sci., 2014, 7, 1564–1596.
36. M. Yu, Y. Huang, C. Li, Y. Zeng, W. Wang, Y. Li, P. Fang, X. Lu and Y. Tong, Adv. Funct. Mater., 2015, 25, 324–330.
37. H. Zhou, X. Yang, J. Lv, Q. Dang, L. Kang, Z. Lei, Z. Yang, Z. Hao and Z.-H. Liu, Electrochim. Acta, 2015, 154, 300–307.
38. Y. Yang, Y. Hao, J. yuan, L. Niu and F. Xia, Carbon, 2015, 84, 174–184.
39. X. Xu, F. Xia, L. Zhang and J. Gao, Sci. Adv. Mater., 2015, 7, 423–432.
40. L. Ma, X. Shen, H. Zhou, Z. Ji, K. Chen and G. Zhu, Chem. Eng. J., 2015, 262, 980–988.
41. Y. Liu, Z. Cheng, H. Sun, H. Arandiyan, J. Li and M. Ahmad, J. Power Sources, 2015, 273, 878–884.
42. M. Li, B. Dong, G. Gao and S. Ding, Mater. Lett., 2015, 155, 30–33.
43. X. Li, J. Shen, N. Li and M. Ye, Mater. Lett., 2015, 139, 81–85.
44. Q. Ke, Y. Liao, S. Yao, L. Song and X. Xiong, J. Mater. Sci., 2016, 51, 2008–2016.
45. N. I. Kovtyukhova, P. J. Ollivier, B. R. Martin, T. E. Mallouk, S. A. Chizhik, E. V. Buzaneva and A. D. Gorchinskiy, Chem. Mater., 1999, 11, 771–778.
46. X. A. Chen, X. Chen, F. Zhang, Z. Yang and S. Huang, J. Power Sources, 2013, 243, 555–561.
47. D. Cheng, Y. Yang, J. Xie, C. Fang, G. Zhang and J. Xiong, J. Mater. Chem. A, 2015, 3, 14348–14357.
48. H. Quan, B. Cheng, Y. Xiao and S. Lei, Chem. Eng. J., 2016, 286, 165–173.
49. K. Krishnamoorthy, M. Veerapandian, K. Yun and S. J. Kim, Carbon, 2013, 53, 38–49.
50. Y. Xu, K. Sheng, C. Li and G. Shi, ACS Nano, 2010, 4, 4324–4330.
51. M. C. Liu, L. Kang, L. B. Kong, C. Lu, X. J. Ma, X. M. Li and Y. C. Luo, RSC Adv., 2013, 3, 6472–6478.
52. J. I. Paredes, S. Villar-Rodil, P. Solis-Fernandez, A. Martinez-Alonso and J. M. Tascon, Langmuir, 2009, 25, 5957–5968.
53. L.-S. Zhang, W. Li, Z.-M. Cui and W.-G. Song, J. Phys. Chem. C, 2009, 113, 20594–20598.
54. D. Ghosh, S. Giri and C. K. Das, Nanoscale, 2013, 5, 10428–10437.
55. A. Maione and M. Devillers, J. Solid State Chem., 2004, 177, 2339–2349.
56. W. Li, L.-S. Zhang, Q. Wang, Y. Yu, Z. Chen, C.-Y. Cao and W.-G. Song, J. Mater. Chem., 2012, 22, 15342–15347.
57. S. Min, C. Zhao, Z. Zhang, G. Chen, X. Qian and Z. Guo, J. Mater. Chem. A, 2015, 3, 3641–3650.
58. B. Dong, H. Zhou, J. Liang, L. Zhang, G. Gao and S. Ding, Nanotechnology, 2014, 25, 435403.
59. H. Liu, J. Zhang, D. Xu, B. Zhang, L. Shi, L. Huang and S. Tan, Appl. Surf. Sci., 2014, 317, 370–377.
60. Y. Li, H. Wang, J. Jian, Y. Fan, L. Yu, G. Cheng, J. Zhou and M. Sun, RSC Adv., 2016, 6, 13957–13963.
61. T. Liu, H. Chai, D. Jia, Y. Su, T. Wang and W. Zhou, Electrochim. Acta, 2015, 180, 998–1006.
62. G. K. Veerasubramani, K. Krishnamoorthy and S. J. Kim, RSC Adv., 2015, 5, 16319–16327.
63. H. Zhang, Y. Chen, W. Wang, G. Zhang, M. Zhuo, H. Zhang, T. Yang, Q. Li and T. Wang, J. Mater. Chem. A, 2013, 1, 8593–8600.
64. X. Li, Y. Feng, M. Li, W. Li, H. Wei and D. Song, Adv. Funct. Mater., 2015, 25, 6858–6866.

---